U.S. patent number 3,908,641 [Application Number 05/477,255] was granted by the patent office on 1975-09-30 for electrocardiograph with improved stylus control circuits.
This patent grant is currently assigned to The Birtcher Corporation. Invention is credited to Donald William Judson, Glenn Roy Tormey, Jr..
United States Patent |
3,908,641 |
Judson , et al. |
September 30, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Electrocardiograph with improved stylus control circuits
Abstract
An electrocardiograph unit employing a marker stylus and a
writing stylus for recording signals on heat sensitive paper is
disclosed. The writing stylus, upon operation of a run switch, is
heated first by an immediate burst of high energy heating current
and then is heated by a predetermined threshold level of heat
current. The marker stylus receives continuous heat and upon
operation of the run switch, automatically generates a series of
time reference marks by momentary movements into and out of contact
with the paper. An operator can selectively open and close an
encoding switch which overrides the automatic timing feature and
commands the marker stylus to encode "Morse Code type" signals on
the paper. These coding signals are indicative of the particular
cardiac waveforms then being written on the paper by the writing
stylus.
Inventors: |
Judson; Donald William (Simi
Valley, CA), Tormey, Jr.; Glenn Roy (San Fernando, CA) |
Assignee: |
The Birtcher Corporation (Los
Angeles, CA)
|
Family
ID: |
23895174 |
Appl.
No.: |
05/477,255 |
Filed: |
June 7, 1974 |
Current U.S.
Class: |
600/523;
346/33ME; 346/139C; 346/76.1; 346/51 |
Current CPC
Class: |
A61B
5/333 (20210101) |
Current International
Class: |
A61B
5/0432 (20060101); A61B 005/04 () |
Field of
Search: |
;128/2.5Q,2.6BF,2.6R,2.6V ;346/33ME,51,52,54,57,76R,87,139C,143
;219/216 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Jackson & Jones
Claims
What is claimed is:
1. An electrocardiograph unit having means for receiving and
amplifying signals derived from electrodes placed on a patient's
body, a strip chart recorder supplied with heat sensitive paper and
a waveform stylus for recording said waveforms in visible form, the
improvement comprising:
a marker stylus provided with continuous heat when the
electrocardiac unit is in operation;
means for automatically moving said marker stylus into and out of
contact with said paper, thereby applying a time reference mark on
the heat sensitive paper; and
means, responsive to an operator for overriding said automatic
maker stylus moving means and moving said marker stylus into and
out of contact with said paper under control of the operator,
thereby supplying encoded marks indicative of the
electrocardiograph waveform then being written on said paper by
said waveform stylus.
2. An electrocardiograph in accordance with claim 1 and further
comprising:
a run switch operable for bringing the heat sensitive paper from a
stopped position to a selected speed; and
means for applying an initial surge of heating current in excess of
a predetermined level to said waveform stylus in response to
operation of the run switch.
3. An electrocardiograph in accordance with claim 2 wherein:
said heat applying means includes means for automatically reducing
the heating current to said predetermined level immediately
following application of said initial surge of heating current to
said waveform stylus.
4. An electrocardiograph in accordance with claim 2 wherein said
heating current applying means comprises:
an amplifier receiving an input voltage upon operation of said run
switch;
an electrical coil for heating the waveform stylus; and
means connecting said coil in a negative feedback loop for said
amplifier.
5. An electrocardiograph in accordance with claim 4 and further
comprising:
means connecting a source of potential through said run switch to
an input terminal of said amplifier for initially supplying a high
spike voltage followed by a lower steady state voltage to said
amplifier.
6. An electrocardiograph in accordance with claim 5 wherein said
connecting means comprises:
a resistance/capacitor circuit connected between the potential
source and the input terminal of said amplifier.
7. An electrocardiograph unit having means for receiving and
amplifying signal combinations from electrodes placed on a
patient's body and a strip chart recorder supplied with heat
sensitive paper for recording waveforms indicative of various
signal combinations, the unit comprising:
a heated waveform stylus for recording individual waveforms
indicative of signal combinations selected by an operator;
a run switch operable for bringing the heat sensitive paper from a
stopped position to a selected speed; and
means for applying heat by applying an initial surge of heating
current in excess of a predetermined level to said waveform stylus
in response to operation of the run switch, said heat applying
means including means for automatically reducing the heating
current to said predetermined level immediately following
application of said initial surge of heating current to said
waveform stylus.
8. An electrocardiograph in accordance with claim 7 wherein said
heat applying means comprises:
a negative feedback amplifier receiving an input potential upon
operation of said run switch; and
a heater coil for said stylus connected in the feedback circuit of
said amplifier.
9. An electrocardiograph in accordance with claim 8 and further
comprising:
a marker stylus provided with continuous heat when the
electrocardiograph unit is in operation;
means for automatically moving said marker stylus into and out of
contact with said paper, thereby applying a time reference mark on
the heat sensitive paper; and
manually operative means overriding said automatic marker stylus
moving means for supplying encoded marks indicative of the
electrocardiograph waveform then being recorded on said paper by
said marker stylus.
10. An electrocardiograph in accordance with claim 7, further
comprising:
means for increasing the initial surge of heating current supplied
by said heat applying means when a faster paper speed is utilized.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to medical instrumentation and more
particularly relates to electrocardiograph testing apparatus for
measuring a patient's cardiac waveforms.
2. Description of the Prior Art
Electrocardiograph units for depicting signals indicative of the
heart activity on heat sensitive strip chart recorder paper are
well known. Different signal waveforms associated with a patient's
heart are analyzed by one skilled in the art of electrocardiogram
analysis. Various signal combinations electrically depict the
heart's condition based primarily upon its location within the
patient's chest cavity. It is essential, for proper analysis of the
waveforms recorded on the strip chart recorder paper, that the
particular waveform being analyzed is properly identified.
Successful analysis also requires that a time reference base on the
paper be available so that various portions of the waveform can be
correlated with time. Correlation of the time reference base and
supplying a label identifying each of the individual waveforms has
proven to be a difficult problem in the prior art devices.
For example, in prior art devices, most devices do not provide any
means for labeling the waveforms prior to the time that they are
recorded on the paper by the writing stylus. In such devices it is
left to the operator to remember to manually place a label on the
paper after the various traces are obtained. Incorrect labeling or
no labeling at all is the inevitable result for many tests
employing such prior art devices.
In known prior art devices, an operator pushes a run button, which
button, when operated, starts the strip chart recorder paper in
motion. Heat in such devices is continually applied to the writing
stylus for tracing the cardiac waveforms. The stylus must receive a
specific amount of heat in order to achieve the desired darkness
for the various cardiac traces. Because the paper is stationary
until the operator starts a run, the stylus creates a large
darkened area at the beginning of the trace. In some instances of
high stylus heat (for a very dark trace) the stylus has been known
to burn the paper. Such devices create a potentially dangerous fire
hazard; and in many instances totally obliterate the beginning
portion of each cardiac waveform trace.
In an effort to avoid the above-noted problems associated with the
prior art devices, the operation of a run button was employed to
trip a delay circuit. The delay circuit attempted to prevent the
recorder paper from starting in motion until a heat circuit could
slowly warm the writing stylus to a proper temperature. This
technique has not proven successful because the operator in many
cases believes the machine is not operating properly due to the
delay. As a result the operator takes "corrective" action by
pushing other buttons, manually moving the paper etcetera all to
the detriment of the unit. Furthermore, the delay time at the
initiation of a trace has created complex timing problems in
attempting to analyze waveforms relative to a time base.
The above-noted disadvantages of the prior art are overcome by the
principles of this invention wherein an electrocardiograph unit
includes a normally cold writing stylus and a heat control circuit
for immediately applying a high energy heat signal to
instantaneously heat the stylus when the run button is operated. A
marker stylus provides a dual function of recording encoded signals
indicative of the cardiac waveform being traced together with an
accurate time reference base.
SUMMARY OF THE INVENTION
In the electrocardiograph unit of this invention, a marker stylus
is employed as is a writing stylus. The marker stylus is provided
with continuous heat when the electrocardiograph unit is in
operation. It is positioned typically along a margin of the paper
and out of contact with the strip chart recorder paper. A timing
device and a solenoid repetitively move the marker stylus into and
out of contact with the paper in order to supply a time reference
mark along the paper's margin.
Each particular cardiac signal which is being written by the
writing stylus during operation of the cardiograph unit is
identified in a simple and reliable manner in this invention. An
operator merely closes and opens an encoding switch to designate
the particular cardiac waveforms in question by Morse Code type
marks. Closure of the encoding switch causes the timing device to
be momentarily overridden until the marker stylus has finished
applying the encoded marks indicative of the particular
electrocardiogram waveform then being written on the paper by the
waveform stylus. After the encoding operation is completed the
encoding switch is released and the timing circuit automatically
resumes control over the marker stylus to record a time reference
on the paper.
In the electrocardiograph unit of this invention, operation of the
run switch immediately starts to strip chart recorder paper and the
writing stylus heat control circuit. The heat control circuit
includes a negative feedback signal processing circuit which
responds to the application of a run voltage by automatically
supplying a high burst of electrical current to the writing stylus.
The burst is selected in magnitude and duration so that it
imediately heats the writing stylus to a desired temperature. The
high heat burst is present for a short duration, and then the heat
to the writing stylus automatically achieves a predetermined level
which is thereafter maintained.
The electrocardiograph unit of this invention accomplishes the
above-noted features for at least two distinct paper speeds. Such
paper speeds are accommodated by impedances which are inserted or
removed from the negative feedback portion of the writing stylus
heat control circuit.
The principles of this invention thus provide an electrocardiograph
unit having a marker stylus for accomplishing the dual function of
(a) supplying encoded marks indicative of the electrocardiograph
waveform being traced out on the recorder paper by the waveform
stylus, and (b) supplying an accurate time reference mark after the
encoding marks. The writing stylus of the electrocardiograph unit
of this invention is maintained in a cold condition until the
operator presses a run button. At that moment, a burst of current
supplies heat to immediately bring the writing stylus to a writing
temperature, which heat is automatically controlled by writing
stylus temperature control circuitry to thereafter hold the writing
stylus at a predetermined heat.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an improved electrocardiograph system
in accordance with the principles of this invention;
FIG. 1A is a diagram symbolically illustrating well known cardiac
vector locations and cardiac waveform shapes;
FIG. 2 is a detailed circuit schematic of the marker stylus control
circuit shown in block form in FIG. 1; and
FIG. 3 is a detailed circuit schematic of the writing stylus heat
control circuit shown in block form in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning now to FIG. 1, a series of electrodes 10 through 14 are
shown. The electrodes are appropriately labeled to indicate the
location of attachment of such electrodes to the body of a patient
whose electrocardiograph is to be measured. It is well known in the
art of biophysical measurements to clean the electrodes prior to
their application to any suitable part of a patient's right leg,
left leg, right arm, left arm and several predetermined chest
locations.
A patient's heart generates electrical activity which establishes
potentials at the surface of the patient's body. An
electrocardiograph receives signals indicative of the electrical
potentials at the various electrodes. Those received signals are
processed and the electrocardiograph records waveforms which
display the difference in various combinations of the potentials
sensed by electrodes 10 through 14. During signal processing,
output signals from the electrodes are applied to signal amplifiers
and switching circuits shown in block form as element 25 in FIG. 1.
Because various electrode combinations are required to electrically
depict the heart's condition, it is standard in the art to provide
a plurality of switches which interconnect in a known manner, the
individual signals sensed by the various electrodes.
Each heart measurement is depicted on a display by a signal
waveform which is related to the potentials measured at the various
electrodes on the patient's body as compared to certain other
measured potentials. These known techniques are fully described for
example in the Tektronics First Edition of "Biophysical Measures"
copyrighted 1970, Tektronics, Inc., Beaverton, Ore.
In the referenced article, at pages 49 through 59, a plurality of
output signals known in the art as Lead No. 1 ECG, Lead No. 2 ECG,
Lead No. 3 ECG, aV.sub.R, aV.sub.L, and aV.sub.F and V are
described. The Tektronics reference fully describes, in a detailed
analysis, the various designations and signal combinations required
for presentation of displayed signals representing these output
signals. Briefly, however, the electrical potential within the
heart is projected along planes such as a frontal plane on the
chest of the patient's body as depicted in FIG. 1A. Lead No. 1 ECG
is the waveform which depicts a vector, known in the art as an R
wave, from the right arm to the left arm.
Referring to FIG. 1, switch 26 connects electrode 10 associated
with the right arm to electrode 13 associated with the left arm so
as to supply an output on Lead No. 1. The output signal is a
potential difference which is referred to in the art as an R wave
of Lead No. 1 ECG. Other switches (not shown) in a well known
manner connect the other various input electrodes as is required to
generate other R waves for the other output leads depicted in FIG.
1.
FIG. 1A depicts the various cardiac vectors together with
illustrative sample waveforms of the relative amplitude and angular
position of the cardiac vectors as are normally presented on an
electrocardiograph display. Such a display, for example, may
typically be a strip chart recorder 50, FIG. 1.
Output signals from the various cardiac R waves are supplied as
analog signals to an amplifier 27, the output of which is connected
to an optical diode 30. It is essential, of course, that a patient
be electrically isolated from current from the electrocardiograph
equipment that generates and displays the waveforms indicative of
the various R waves. Such electrical isolation is achieved by
utilizing optical coupling between diode 30 and a photosensitive
diode 31. Similarly, power for the unit is divided into two
portions 35 and 38 which are isolated from each other by a
transformer coupling 36. This optical and electrical isolation is
fully described and claimed in U.S. Pat. No. 3,808,502 issued Apr.
30, 1974 entitled "ISOLATOR CIRCUIT FOR USE WITH ELECTRICAL MEDICAL
EQUIPMENT" by Algis John Babilius and assigned to the same assignee
as the present application. The optical and electrical isolation
thus need not be described in further detail herein.
Source 38 supplies power to a marker stylus control circuit 40, a
writing stylus heat control circuit 45 and to a strip chart
recorder motor 54. Power source 38 also supplies heating current to
the stylus arms of two galvanometers G.sub.1 and G.sub.2 identified
as elements 65 and 70 in FIG. 1.
Optical signals coupled between diode 30 and diode 31 are converted
to electrical signals for amplification by a standard high-gain
amplifier 36. Amplified signals from amplifier 36 are employed to
drive a galvanometer 65. Galvanometer 65 includes a writing stylus
65A. Stylus 65A is in continuous contact with strip chart recorder
paper 50. When the unit is in operation by closure of run switch
220, stylus 65A forms a trace of the particular R wave then being
measured by the operator.
Paper 50 is chemically treated heat sensitive recording paper of
any well known type. Such paper responds to the heat of stylus 65A
to form a dark trace on paper 50. If heat were continually applied
to stylus 65A by way of heat coil 65B, as in the prior art, there
is a good likelihood that the paper might catch on fire or make
large darkened areas that obliterate the beginning portions of the
waveform as the paper 50 starts from a stopped position.
In accordance with the principles of this invention, upon activity
of the run switch, the writing stylus heat control circuit
automatically applies a high energy pulse to rapidly heat the
writing stylus 65A and thereafter hold the stylus heat at a
predetermined amount. The problems created by electrocardiograph
operation is further compounded by different paper speeds as
required in different tests. In accordance with the principles of
this invention, the various paper speeds are automatically
compensated for in that the paper speed switch 221 automatically
provides predetermined increases in heat as the paper speed is
measured.
In order to interpret the strip chart recorder, it is necessary
that time indications be provided on the strip chart recorder paper
50. These time indications are provided by galvanometer 70, which
galvanometer is normally positioned out of contact with paper 50.
Galvanometer 70 is driven into momentary contact with paper 50 by
solenoid 71 under control of the marker stylus control circuit 40.
Stylus 70A of galvanometer 70 is continually heated by the power
source 38 by passage of current through a heating coil 70B which
wraps around stylus 70A to supply heat thereto in a well known
manner.
Constant heat of a suitable amount supplies time dots via stylus
70A of galvanometer 70 each time that solenoid 71 operates. Marker
control circuit 40 activates solenoid 71 repetitively based upon a
predetermined timing period. Inasmuch as the marker stylus 70A only
touches the paper momentarily, there is no fear of burning the
paper or creating fires during use of the instrument.
FIG. 2 depicts the circuit details for the marker control circuit
40 of FIG. 1. Power source 38 is transformer coupled in a standard
manner to an open run switch 220. When run switch 220 is closed by
an operator, a suitable voltage is applied through a run signal
lead to a speed control and motor circuit 49 so as to drive the
strip chart recorder paper 50, FIG. 1.
In its normal open position, run switch 220 supplies a back-bias
voltage to the junction of resistor 215 and diode 225. With diode
225 back-biased, transistor 235 is maintained in a normally
non-conductive state. Closure of run switch 220 simultaneously
supplies a run signal to the strip chart recorder and removes the
back-bias from transistor 235.
The base of a transistor 245 is connected via resistor 246 to an
output terminal of a timer 255. Timer 255 may be any suitable timer
which emits a forward bias signal of a given duration once for each
predetermined time interval. Such a signal is applied to transistor
245 on a repetitive basis. Forward bias on the base of transistor
245 causes it to be conductive. During the time transistor 245
conducts, transistor 235 is accordingly rendered conductive. With
transistor 235 conductive a current flow path from voltage source
275 through solenoid 71 is established to ground through transistor
235.
Solenoid 71, when actuated, causes stylus 70A of galvanometer 70 to
move into and out of contact with the strip chart recorder paper
50. As mentioned earlier in conjunction with FIG. 1, stylus 70A is
continuously supplied with heating current from source 38. The
momentary contact with paper 50 by stylus 70A repetitively records
a timing mark on the strip chart recorder paper.
Timer 255 is continually supplied with power during the time that
the electrocardiograph unit is in operation. Timer 255 may, for
example, be an integrated circuit signetics modem NE/SE 555 having
an output terminal 3, a reset terminal 4, a trigger terminal 2,
threshold terminal 6, and discharge terminal 7. Terminals 7, 6 and
2 are connected to a voltage divider between ground and source 258,
which divider includes resistors 259, 260 and 261. A capacitor 262
is charged and discharged to repeatedly activate timer 255 in a
well known manner.
In accordance with this invention, timer 255 may be overridden by
closure of an encoding switch 265. Encoding switch 265 is under
manual command of an operator. Closure of switch 265 applies ground
to reset terminal 4. Ground resets timer 255 and also supplies a
signal to solenoid 71 for the time duration that switch 265 is
closed. In accordance with a preferred aspect of this invention,
the operator may employ switch 265 to record a series of marks on a
margin of the chart paper 50, which marks identify each cardiac
vector being traced by the writing stylus 65A at that moment in
time. For example, the Lead No. 1 may be indicated by a dot i.e. a
momentary closure of switch 265. Lead No. 2 may be indicated by two
dots, Lead No. 3 by three dots. The waveform aV.sub.R may be
indicated by one long dash, i.e. a longer closure of switch 265. A
combination of dots and dashes may be employed to identify the
several well known V positions as sensed by electrode movements to
designated locations across the patient's chest. Switch 265 after
conclusion of the encoding operation is returned to its normal
position as shown in FIG. 2. That position allows timer 255 to
reset. Thereafter one second time frame marks are recorded via
solenoid 71 on the strip chart recorder paper 50 in the manner
earlier described.
FIG. 3 is a detailed circuit schematic of the writing stylus heat
control circuit 45. As originally noted in FIG. 1, the run switch
controls both the writing stylus heat control circuit 45 of FIG. 3
and the marker stylus heat control circuit 40 of FIG. 2. With run
switch 220 open, control circuit 45, FIG. 3, does not apply any
heat to the writing stylus 65A of galvanometer 65. Waveform stylus
65A is in continuous contact with the strip chart recorder paper
50, FIG. 1. Because the stylus 65A is cold, and the paper is heat
sensitive, the paper does not respond when run switch 220 is open.
Accordingly, no marks are written on paper 50.
It is imperative, for the reasons noted earlier, that when the run
switch 220 is closed that stylus 65A receive sufficient heat to
immediately come to a proper writing temperature. Thereafter,
stylus 65A must hold its temperature at an amount suitable to make
a trace of desired darkness on the paper for the paper's given
speed. In accordance with the principles of this invention, the
foregoing features are accomplished by the heat control circuitry
45 of FIG. 3.
In operation, closure of the run switch 220, FIG. 3, applies an
immediate power burst to stylus 65A, and then holds steady state
power at a predetermined level as selected by an operator. As shown
in FIG. 3, closure of the run switch 220 applies a potential from
source 315 to an RC network 328. The RC network 328 comprises a
series circuit of capacitor 321 and resistor 322, which series
circuit is connected in parallel with a resistor 325. The parallel
circuit is further connected in series between source 315 and
ground by a potentiometer 335. If the RC network 328 were not
present, resistor 325 and potentiometer 335 would act as a simple
voltage divider. A voltage divider for the input to the operational
amplifier 340 would simple provide a predetermined output signal
according to the selected position of potentiometer adjustment
329.
The voltage input to the operational amplifier 340, because RC
network 328 is present, drops immediately to a low level when run
switch 220 is closed. Waveform 350 depicts the voltage waveform at
junction 329. Assume that source 315 is -15 volts. As depicted by
waveform 350, at time t.sub.o when switch 220 is closed, the
voltage at junction 329 originally drops to a negative voltage in
the order of -9 volts. As capacitor 321 charges, in series with
resistor 322 and resistor 325 in parallel, the voltage as shown by
waveform 350 rises to a more positive level 351 of approximately -4
volts.
Input waveform 350 is applied to an operational amplifier 340,
which, in turn, is connected to a Darlington amplifier 360. The
circuit further includes transistors 355, 356 and a negative
feedback loop connected between the emitter of transistor 356 and
an input terminal of operational amplifier 341. In response to the
input waveform 350 the circuit of FIG. 3 develops an output heating
voltage, waveform 375 for stylus 65A. Output voltage waveform 375
is essentially the inverse of waveform 350. As shown by waveform
375, the voltage of initially 0 volts, at time t.sub.o, raises
abruptly to a spike voltage of approximately 6 volts. After about
one-quarter of a second the signal decays to a nominal value of
approximately 4 volts.
Current flowing in the feedback loop flows through heater element
65B which surrounds stylus 65A. The high current flow from the
spike voltage of waveform 375 immediately heats stylus 65A. The
feedback loop nulls out the input voltage to operational amplifier
340. In approximately one quarter of a second the circuit
stabilizes and holds the heating current at a constant level for
the duration of a run.
Different operators desire different darkness for the trace by
stylus 65A. For this reason, potentiometer 329 is adjustable so
that the input signal to operational amplifier 340 is variable.
Because the output signal at heater 65B is a function of the input
signal for amplifier 340, the output at waveform 375 is also
adjustable. Thus, the height and width of the spike voltage at
waveform 375 is subject to adjustment by an operator. The time
duration of the initial energy burst is determined by selecting
different values for the input to the operational amplifier 340. In
addition, the resistance in the feedback circuitry of the control
circuit of FIG. 3 may be varied depending upon the condition of
switch 385.
Paper speed switch 385 is connected across resistor 379. As shown
in FIG. 3, paper speed switch 385 is in a closed condition
indicative of a slow paper speed. Closed switch 385 bypasses
resistor 379 removing it from the feedback loop.
In the event that the operator desires a faster paper speed, it is
necessary to increase the heating current for writing stylus 65A.
Opening the paper speed switch 385 adds resistor 379 in series with
resistor 378 and thus develops a larger feedback resistance. In
accordance with well known feedback theory, a larger feedback
resistance causes a larger output voltage. Accordingly, heater coil
65B develops more initial heat for writing stylus 65A. The darkness
of the waveform on the recorder paper 50 is adjusted in the manner
described, to be essentially uniform for either of the two noted
paper speeds. Whereas the feedback circuity has been described as
involving only two paper speeds, it will be readily apparent to
those skilled in the art that additional paper speeds can be
accommmodated. Additional switches and feedback resistors may
selectively short out or add resistance in the feedback loop as is
required for several different paper speeds.
Paper for a strip chart recorder 50 is normally layed out in
millimeter grids. Typical paper speeds are 25 millimeters per
second or 50 millimeters per second. A stylus such as Birtcher
Catalog No. 307 may be employed in the electrocardiograph of this
invention. In such an instance the following values may be employed
for the heater control circuit of FIG. 3:
Resistor 325 18 K ohms Capacitor 321 680 uf Resistor 322 20 K ohms
Resistor 345 72 K ohms Transistors 355 and 356 IC 306 or Type MJE
720 Operational Amplifier IC 303/741 Resistor 378 27 K ohms
Resistor 379 5.6 K ohms
It is to be understood that the foregoing features and principles
of this invention are merely descriptive, and that many departures
and variations thereof are possible by those skilled in the art,
without departing from the spirit and scope of this invention.
* * * * *